Spatial frequency channel models have been used to explain a broad range of phenomena in human vision involving the detection and appearance of spatial patterns. Contrast detection and matching experiments with extended periodic patterns have shown narrow spatial frequency tuning while contrast detection of localized, aperiodic patterns and masking of periodic patterns have shown broader tuning. We have found that the former result is typical of situations involving apparent contrast changes while the latter result is characteristic of changes of apparent form. We will do a variety of experiments to explore the correlation of spatial frequency interaction with suprathreshold perceptual attributes of stimuli and with perceptual task performed. The experiments contribute directly to basic understanding of human visual function and are relevant to clinical diagnostic use of detection tasks and to diagnostic radiology.